The proposed research Aims to investigate the genetic and biochemical mechanisms of competence regulation in Streptococcus mutans by the newly discovered high cell density responsive hdrRM operon. The goals of this project will be achieved in two Aims by: 1) Determining the steps of the competence cascade that are regulated by hdrRM 2) Determining the mechanism of gene regulation by HdrRM. 1) To analyze how the hdrRM operon interacts with the genes of the competence cascade, a combination of transcription, epistatic, and mutation analysis will be performed. These studies will provide the genetic basis for the role of hdrRM as a negative regulator of competence. 2) To characterize the mechanism of HdrR as a regulator of competence, HdrR will be used for DNA binding assays of competence gene promoters. In addition, in vivo protein interaction studies between HdrR and HdrM will characterize possible interactions between the two proteins. These data are expected to provide biochemical evidence for the genetic and transcriptional data generated in Aim 1 as well as determine the mechanism of signal transduction in the HdrRM system. The goal of this project is to use competence regulation as a model for the mechanism by which the hdrRM operon is able to regulate other important virulence factors of Streptococcus mutans. Since this operon is strongly induced by conditions of high cell density, it is expected that it will play a crucial role for both survival and pathogenicity in the high cell density environment of the dental plaque. Project Narrative: Dental caries is the most common human bacterial disease and is largely correlated with the overgrowth of Streptococcus mutans. Since its ability to cause disease requires it to function in the high cell density conditions of dental plaque, studies of gene regulators that function in high cell density conditions may yield novel treatment strategies to control the pathogenesis of S. mutans.